p-Hydroxyphenylpyruvate dioxygenase (HPPD; p-hydroxyphenylpyruvate: oxygen oxidoreductase, EC 1.13.11.27) in plant tissues is an enzyme central to the biosynthesis of the quinoid compounds derived from the amino acid tyrosine, such as plastoquinones or tocopherols. During the biosynthesis of quinone, tyrosine is first transaminated by a transaminase to give p-hydroxyphenylpyruvate, which is then decarboxylated and hydroxylated by the enzyme HPPD in a complex sequence. The product of this reaction is homogentisic acid. These first two steps of quinone biosynthesis can analogously also be detected in animal tissues and microorganisms. While, in plants, the homogentisic acid formed can be reacted to give plastoquinones and tocopherols, however, it is an intermediate in the catabolism of the amino acid tyrosine in animals and microorganisms. Plastoquinones and tocopherols are essential structures for plants. Inhibitors of the biosynthesis of plastoquinones and tocopherols should therefore be potential herbicides.
Catabolic p-hydroxyphenylpyruvate dioxygenases have previously been purified and characterized from animal tissues (Lindstedt, S. and Odelhxc3x6g, B. (1987) Meth. Enzymol. 142, 139-142) and microorganisms (Lindstedt, S. and Odelhxc3x6g, B. (1987) Meth. Enzymol. 142, 143-148). (Anabolic) p-hydroxyphenylpyruvate dioxygenases from plants, in contrast, have been described in the literature (Fiedler, E., Soll, J. and Schultz, G. (1982) Planta 155, 511-515), but no method for enriching the enzyme has been described to date.
Specifically, there has been no information to date in the literature on simple methods of measuring the plant enzyme which, of course, are indispensable for purification of the enzyme and detecting inhibitors of the enzyme.
The method described by Fiedler et al. is relatively complicated and does not allow the quantitative assays to be carried out which are required for identifying potential herbicides.
What follows describes a method for enriching the anabolic p-hydroxyphenylpyruvate dioxygenases from plant tissues and a method by means of which the enzymatic activity of the enzyme can be measured in a simple manner without complete purification of the enzyme being necessary.
The invention therefore relates to:
A method for enriching an anabolic p-hydroxyphenylpyruvate dioxygenase from plant cells, which comprises isolating the enzyme directly from a buffer in which the cells were homogenized.
The invention furthermore relates to an assay system for identifying inhibitors of p-hydroxyphenylpyruvate dioxygenase from plants, which comprises incubating an enriched HPPD from plants with a test substrate to be examined and determining the enzymatic activity of the enzyme in comparison with the activity of the uninhibited enzyme.
In particular, the invention relates to a method in which the plant tissue or cells are homogenized in extraction buffer, the extract is filtered, the supernatant is subjected to fractional ammonium sulfate precipitation, and the precipitate formed is redissolved.
Suitable extraction buffers are the extraction buffers conventionally used for plant cells, in particular the extraction buffer which is composed as follows:
20 mM phosphate buffer pH 7.0;
0.14 M KCl;
0.1 mg/ml glutathione and
1% of insoluble polyvinylpyrrolidone.
Suitable test substrates are all compounds which are potential HPPD inhibitors. These potential inhibitors are preferably stucturally similar to the natural substrate of HPPD. However, other substances which do not bind in the active center of the enzyme, but which inhibit the enzyme in a different manner, can also be used.
The enzymatic activity can be determined by means of the methods known from the literature (see Bergmeyer, H. U., Methoden der enzymatischen Analyse [Methods in enzymatic analysis], Volumes 1 and 2, Verlag Chemie, Weinheim, (1974) and Suelter, C. H., Experimentelle Enzymologie: Grundlagen fxc3xcr die Laborpraxis [Experimental Enzymology: Fundamentals of Laboratory Practice], Fischer Stuttgart (1990)).
A modified form of the method for measuring catabolic HPPDs from human liver has been described by Lindblad (Lindblad, B. (1971) Clin. Chim. Acta 34, 113-121). End-point measurements are used in this case for detecting 14Cxe2x80x94CO2, which is liberated from 14C-p-hydroxyphenylpyruvate by the enzymatic activity of HPPD. If HPPD inhibitors are present in the reaction batch, the enzyme reaction and hence the liberation of CO2 are suppressed. This assay method allows inhibitors of the enzyme to be found.
It is preferred to carry out the assay in such a way that the enzymatic activity of the HPPD is started up after preincubation of the enriched HPPD with the potential inhibitor by adding the radiolabeled 14C-p-hydroxyphenylpyruvate, stopping the reaction after a suitable incubation time, and measuring the enzymatic activity indirectly via the radioactivity which has been liberated.
It is self-evident that the assay according to the invention can also be carried out with the purified enzyme. The enzyme can be further enriched in a manner known per se by means of chromatography on anion exchangers, such as, for example, Q-Sepharose, by Pharmacia, followed by gel permeation chromatography, such as, for example, Superdex 200, by Pharmacia, but this is not necessary for carrying out the assay.
Surprisingly, a new class of HPPD inhibitors has now also been identified using this assay system. They are herbicides from the group of the 2-benzoylcyclohexane-1,3-diones. This is surprising since an entirely different mechanism of action is suggested in the literature for this compounds class on the basis of its herbicidal symptoms (they lead to bleaching of the plants). It is assumed that, analogously to other bleaching herbicides, they inhibit phytoene desaturase (Soeda, T. and Uchida, T. (1987) Pestic. Biochem. Physiol. 29, 35-42; Mayonada, D. J., Hatzios, K. K., Orcutt, D. M. and Wilson, H. P. (1989) Pestic. Biochem. Physiol. 35, 138-145). It was possible to demonstrate that this compounds class, while not inhibiting phytoene desaturase, does inhibit HPPD; under the assay conditions, 4.5xc3x9710xe2x88x928 M 2-(2-chloro-4-methanesulfonylbenzoyl)-1,3-cyclohexanedione (SC-0051, a typical representative of this class of herbicides) showed a 50% inhibition on the anabolic plant HPPD from maize (which corresponds to the IC50 value). Bleaching herbicides which have a different structure do not, in contrast, inhibit HPPD, but inhibit the enzyme phytoene desaturase.
However, the method according to the invention allows not only the activity of known herbicides to be demonstrated at the molecular level, but also novel, previously unknown herbicidal active substances which inhibit HPPD to be identified.
The invention therefore also relates to the novel herbicidal active substances found by the assay method according to the invention.
The invention therefore also relates to compounds of the formula (I) 
in which
R1 is H, halogen, OH, alkoxy, CN, NO2 or-haloalkyl,
R2 is H, halogen, OH, alkyl, alkoxy, haloalkyl, haloalkoxy or (alkoxy)-carbonyl,
R3 is H, halogen, OH, CN, NO2, haloalkyl, haloalkoxy or R4S(O)mxe2x80x94, where R4 is alkyl and m is zero, one or two,
Q is a radical selected from the group of the formula 
xe2x80x83where
R5, R6, R7, R8 and R9 independently of one another are H, halogen, OH, CN, NO2, SO3H, SO3R4, SO2NR14R15, COOH, COOR4, CONR14R15, Oxe2x80x94COOR4, Oxe2x80x94COR4, -alkyl, alkoxy, haloalkyl or haloalkoxy,
R10, R11 and R12 independently of one another are H, halogen, alkyl, alkoxy, haloalkyl or haloalkoxy,
R13 is H, phenylsulfonyl, alkylsulfonyl or alkylcarbonyl, and
R14 and R15 independently of one another are H, alkyl, aryl or benzyl.
In the formula (I), the radicals alkyl, alkoxy, haloalkyl and haloalkoxy can in each case be straight-chain or branched. Alkyl radicals are, for example, methyl, ethyl, n- or i-propyl or n-, i-, t- or 2-butyl. Aryl embraces aromatic and heteroaromatic radicals, such as, for example, phenyl, 2- or 3-naphthyl or 2-, 3- or 4-pyridyl. Halogen is fluorine, chlorine, bromine or iodine.
The use according to the invention of compounds of the formula (I) in which
R1 is H, halogen, OH, C1-C3-alkoxy, CN, NO2 or C1-C3-haloalkyl,
R2 is H, halogen, OH, C1-C3-alkyl, C1-C3-alkoxy, C1-C2-haloalkyl, C1-C2-haloalkoxy or (C1-C2-alkoxy)-carbonyl,
R3 is H, halogen, OH, CN, NO2, C1-C3-haloalkyl, C1-C2-haloalkoxy or R4S(O)mxe2x80x94, where R4 is C1-C2-alkyl and m is zero, one or two,
Q is a radical selected from the group of the formula 
xe2x80x83where
R5, R6, R7, R8 and R9 independently of one another are H, halogen, OH, CN, NO2, SO3H, SO3R4, SO2NR14R15, COOH, COOR4, CONR14R15, Oxe2x80x94COOR4, Oxe2x80x94COR4, C1-C4-alkyl, C1-C2-alkoxy, C1-C3-haloalkyl or C1-C2-haloalkoxy,
R10, R11 and R12 independently of one another are H, halogen, C1-C3-alkyl, C1-C2-alkoxy, C1-C3-haloalkyl or C1-C2-haloalkoxy,
R13 is H, phenylsulfonyl, C1-C2-alkylsulfonyl or C1-C2-alkylcarbonyl, and
R14 and R15 independently of one another are H, C1-C4-alkyl, phenyl or benzyl,
is of particular interest.
Preferred is the use according to the invention of compounds of the formula (I) with at least one of the following characteristics:
R1 is preferably hydrogen, hydroxyl, fluorine, chlorine, bromine, cyano, nitro, methoxy, ethoxy, difluoromethyl, trifluoromethyl, chloromethyl, trichloromethyl, 2,2,2-trifluoroethyl or pentafluoroethyl,
R2 is preferably hydrogen, fluorine, chlorine, hydroxyl, propyl, ethyl, methyl, methoxy, ethoxy, difluoromethyl, trifluoromethyl, difluoromethoxy, trifluoromethoxy, methoxycarbonyl or ethoxycarbonyl,
R3 is preferably hydrogen, hydroxyl, fluorine, chlorine, bromine, cyano, nitro, difluoromethyl, trifluoromethyl, chloromethyl, trichloromethyl, 2,2,2-tri fluoroethyl, pentafluoroethyl, difluoromethoxy, trifluoromethoxy, methylsulfonyl or ethylsulfonyl;
Q is preferably the radical 
xe2x80x83where
R5, R6, R7, R8 and R9 preferably and independently of one another are hydrogen, fluorine, chlorine, bromine, cyano, nitro, hydroxyl, hydroxysulfonyl, ethoxysulfonyl, ethylaminosulfonyl, methylaminosulfonyl, carboxyl, ethoxycarbonyl, methoxycarbonyl, ethylaminocarbonyl, methylaminocarbonyl, ethoxycarbonyloxy, methoxycarbonyloxy, methylcarbonyloxy, t-butyl, i-propyl, propyl, ethyl, methyl, difluoromethyl, trifluoromethyl, chloromethyl, trichloromethyl, methoxy, ethoxy, difluoromethoxy or trifluoromethoxy,
R10, R11 and R12 preferably and independently of one another are hydrogen, fluorine, chlorine, bromine, ethyl, methyl, difluoromethyl, trifluoromethyl, chloromethyl, trichloromethyl, methoxy, ethoxy, difluoromethoxy or trifluoromethoxy,
R13 is preferably hydrogen, phenylsulfonyl, methylsulfonyl, ethylsulfonyl, methylcarbonyl or ethylcarbonyl, and
R14 and R15 preferably and independently of one another are hydrogen, butyl, propyl, ethyl, methyl or benzyl.
The uses according to the invention of compounds of the formula (I) with a combination of the abovementioned preferred characteristics are also preferred.
The compounds of the abovementioned formula (I) can be prepared, for example, by reacting aromatic carboxylic acid chlorides of the formula (II) 
in which the substituents R1, R2 and R3 are as defined under formula (I), with aromatic or heteroaromatic compounds of the type Hxe2x80x94Q, in which Q is as defined under formula (I), in an inert solvent in the presence of a Lewis acid.
The compounds (I) are preferably prepared in aprotic solvents, in particular in halogenated hydrocarbons, such as, for example, dichloromethane, trichloromethane, 1,2-dichloromethane or 1,1,2,2-tetrachloroethane, at from xe2x88x9220xc2x0 C. up to the boiling point of the reaction mixture, in particular at from room temperature to 50xc2x0 C., the reaction being carried out in the presence of a catalytically to stoichiometrically employed amount of Lewis acid, such as, for example, aluminum chloride, titanium tetrachloride or boron trifluoride.
a) 3-(2,4-Dichlorobenzoyl)-1-phenylsulfonylpyrrole (Table 1, Example No. 150)
4.0 ml of 2,4-dichlorobenzoyl chloride and 4.00 g of aluminum chloride are stirred for 10 minutes at room temperature in 50 ml of 1,2-dichloroethane. A solution of 5.10 g of phenylsulfonylpyrrole in 20 ml of 1,2-dichloroethane is added dropwise to the mixture, and this is stirred for 2 hours at room temperature. The mixture is poured into ice-water and extracted using dichloromethane. The organic phase is dried and evaporated. Recrystallization of the residue from a mixture of ethyl acetate and petroleum ether gives 8.3 g (89% of theory) of 3-(2,4-dichlorobenzoyl)-1-phenylsulfonylpyrrole in the form of white crystals with a melting point of 122xc2x0 C.
b) 2,5xe2x80x2-Dichloro-2xe2x80x2-hydroxy-3xe2x80x2-methyl-4-methylsulfonylbenzophenone (Table 1, Example No. 100)
5.06 g of 2-chloro-4-methylsulfonylbenzoyl chloride are added in portions at room temperature to a mixture of 2.85 g of 4-chloro-2-methylphenol and 5.40 g of aluminum chloride in 100 ml of 1,1,2,2-tetra chloroethane, and the mixture is subsequently refluxed for 7 hours. When cold, the reaction mixture is poured onto 200 g of ice and 500 ml of concentrated hydrochloric acid. The organic phase is separated off, and the aqueous phase is extracted twice using 50 ml of dichloromethane. The combined organic phases are washed until neutral, dried over magnesium sulfate and evaporated. Recrystallization from a mixture of ethyl acetate and diisopropyl ether gives 4.2 g (60% of theory) of 2,5xe2x80x2-dichloro-2xe2x80x2-hydroxy-3xe2x80x2-methyl-4-methylsulfonylbenzophenone in the form of colorless crystals with a melting point of 175-180xc2x0 C.
c) 2-(2,4-Dichlorobenzoyl)-1,4-quinone (Table 1, Example No. 91)
A solution of 2.49 g of 2-(2,4-dichlorobenzoyl)-1,4-dihydroxybenzene in 20 ml of glacial acetic acid is added dropwise at 10xc2x0 C. in the course of 3 minutes to a solution of 1.60 g of sodium dichromate in 2 g of concentrated sulfuric acid and 60 ml of water. The reaction mixture is stirred vigorously for a further 30 minutes at 10-15xc2x0 C. and then filtered. The residue is washed with water until free from acid and recrystallized from diisopropyl ether. 1.74 g (70% of theory) of 2-(2,4-dichlorobenzoyl)-1,4-quinone are obtained in the form of yellowish-orange crystals with a melting point of 100xc2x0 C.
d) 2,2xe2x80x2,4xe2x80x2-Trichloro-6xe2x80x2-hydroxy-4-methylsulfonylbenzophenone (Table 1, Example No. 183)
3.00 g of 2-chloro-4-methylsulfonylbenzoyl chloride are added in portions at room temperature to a mixture of 1.97 g of 3,5-dichlorophenone and 3.23 g of aluminum chloride in 50 ml of 1,1,2,2-tetra chloroethane, and the mixture is subsequently refluxed for 7 hours. When cold, the reaction mixture is poured onto 100 g of ice and 20 ml of concentrated hydrochloric acid. The organic phase is removed, and the aqueous phase is extracted twice using 50 ml of dichloromethane. The combined organic phases are washed until neutral and subsequently extracted three times using in each case 25 ml of 2 N sodium hydroxide solution. The combined alkaline extracts are brought to a pH of 2 using concentrated hydrochloric acid. The precipitate is filtered off, washed with water until neutral and dried. This gives 3.194 g (70% of theory) of 2,2xe2x80x2,4xe2x80x2-trichloro-6xe2x80x2-hydroxy-4-methylsulfonylbenzophenone in the form of colorless crystals with a melting point of 123-126xc2x0 C.
The compounds listed in Table 1 are obtained analogously to the preparation examples described above.
The invention furthermore relates to the use of the herbicidal active substances identified by means of the assay method according to the invention in plant populations.
Suitable plant populations are, in particular, crop plants.